Iron-sulfur protein FIGURE 15.6


Forms of carbon-bonded sulfur.



Organic forms of S represent up to 90% of the total soil S. These forms can be classified into two major types. C-bonded S constitutes up to 30% of the organic S and occurs in amino acids such as cysteine, cystine, and methionine; Fe-S proteins called ferrodoxodins; polypeptides; heterocyclic compounds such as biotin and thiamine, sulfinates, sulfones, sulfonates, and sulfoxides (Fig. 15.6). Sulfate esters are the second group and constitute 30-75% of the organic S in soil. These include sulfate esters (C-O-S), sulfamates (C-N-S), and sulfated thioglycosides (N-O-S) (Fig. 15.7). The measurement of this form involves reduction with hydroioidic acid (HI); thus it is often referred to as Hi-reducible S. This is considered to be the more biologically active or labile form in soil.

Soil microbial biomass S represents 2-3% of the total organic soil S. Microbial S is measured by lysing the cells with chloroform and measuring the S released to a CaCl2 or NaHCO3 extractant (Saggar et al, 1981). Arthrobacter and Pseudomonas contain roughly 10% of their S in an oxidized form regardless of whether they are grown at low or high levels of S substrate (Table 15.7). Fungi such as Fusarium and Trichoderma show a similar percentage when grown at low substrate S levels;

Choline sulfate -O3SO.

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